1. Field of the Invention
The present invention relates to a photocathode.
2. Related Background Art
Photocathodes are used in photodetectors and other measuring devices, and for example, a transmission type photocathode, described in (Patent Document 1: Japanese Published Unexamined Patent Application No. H9-199075) is used. This transmission type photocathode is sensitive to near infrared light and has a window layer, formed of InAlGaAs on a light incidence side of a light absorbing layer, formed of an InGaAs-based material.
With photocathodes, forming of a transparent conductive film on a light incidence side of a light absorbing layer to lower a surface resistance that obstructs analysis of high-speed phenomena, etc., using a photodetector is known (see Patent Document 2: Japanese Published Examined Patent Application No. H4-30706). The provision of a mesh electrode or an island electrode at a light incidence side to apply a bias voltage in a photocathode used in a photodetector is also known (Patent Document 3: Japanese Patent No. 2902708).
Meanwhile, as an application example of a photodetector, fluorescence lifetime analysis, in which a sample is excited by light and a variation in time of an intensity of a fluorescence emitted by the sample is measured, can be cited. A photodetector used for fluorescence lifetime analysis has an electron tube, such as a photomultiplier tube, an image intensifier tube, or a streak tube, in which a photocathode is incorporated. Generally in fluorescence lifetime analysis using a photodetector, a pulsed light of a short wavelength (such as visible laser light) is used as the light for sample excitation and a fluorescence of a longer wavelength than the pulsed light (for example, infrared fluorescence) is measured.